Expandable styrene polymers containing carbon dioxide as blowing agent

ABSTRACT

The invention relates to expandable styrene polymers containing 
     a) a styrene polymer, 
     b) and at least one carbon dioxide absorber, mixed or copolymerized with the above, in an amount of from 0.01 to 50% by weight, based on the sum of a) and b), and 
     c) a blowing agent or blowing agent mixture in an amount of from 0.5 to 12% by weight, 
     where the blowing agent or blowing agent mixture comprises 
     c 1 ) from 10 to 100% by weight of carbon dioxide and 
     c 2 ) from 0 to 90% by weight of at least one further blowing agent. 
     The invention furthermore relates to the styrene polymers containing a carbon dioxide absorber, to the styrene polymer foams obtainable from the expandable styrene polymers, and to an improved process for impregnating styrene polymers with carbon dioxide-containing blowing agents or blowing agent mixtures.

The present invention relates to expandable styrene polymers whichcontain carbon dioxide as blowing agent, and to styrene polymers whichare particularly suitable for this purpose. In particular, the presentinvention relates to expandable styrene polymers which have an increasedretention capacity for carbon dioxide and to the foams obtainable fromthese polymers. Furthermore, the present invention relates to animproved process for impregnating styrene polymers with blowing agentsor blowing agent mixtures containing carbon dioxide.

Polystyrene foams are currently produced using numerous blowing agents,for example aliphatic hydrocarbons and partially or fully halogenatedhydrocarbons. However, in addition to the high costs, these blowingagents have a number of further disadvantages, including flammability(in the case of aliphatic hydrocarbons), a contribution towardsatmospheric pollution and the adverse effect on the ozone layer.

Experiments have therefore been carried out using mixtures of carbondioxide and other blowing agents, such as aliphatic hydrocarbons orfully or partially halogenated hydrocarbons, and pure carbon dioxide hasalso already been used as a blowing agent.

The use of carbon dioxide as a blowing agent in the production ofpolystyrene foams has been known for some time. However, it has hithertobeen employed virtually exclusively as a blowing agent or blowing agentconstituent in the production of polystyrene foams by extrusion.However, the expandable styrene polymers containing carbon dioxide asblowing agent have hitherto had the disadvantage that the carbon dioxideescapes very rapidly from the blowing agent-containing beads.

For example, U.S. Pat. No. 4,911,869 discloses a process for theproduction of moldings from foams in which inorganic gases, such ascarbon dioxide, nitrogen, air, etc., solid carbon dioxide, etc., areused as the blowing agent. As noted in this publication, however, thesegases escape rapidly from the polymer particles. In the processindicated, particles of, inter alia, styrene polymers and copolymers aretherefore first impregnated with carbon dioxide and expanded at atemperature above the glass transition temperature of the polymer/carbondioxide mixture, these expanded particles are then re-impregnated inair, nitrogen or gaseous carbon dioxide or in the presence of liquid orsolid carbon dioxide, etc., and these re-impregnated expanded particlesare subsequently foamed by heating to give foam moldings.

However, this process has a number of disadvantages: a) the carbondioxide-impregnated styrene polymers must be prefoamed immediately andb) the prefoamed polymers are then re-impregnated before the foaming togive moldings. A process of this type is expensive and complicated.

EP-A 411 923 discloses a polystyrene foam which has been foamedexclusively using carbon dioxide, and a process for the production ofthis polystyrene foam. In the process, mixtures of carbon dioxide andthe styrene polymer are extruded and foamed. Foam sheets produced bythis process are suitable, in particular, for applications in the areaof thermoforming.

DE-A 3 923 913 discloses a process for the production of foamed plasticsin which liquid ammonia and/or liquid carbon dioxide is employed as theblowing agent. It was found that, in particular, the foaming ofpolystyrene using liquid carbon dioxide gives excellent results.

EP-A 422 850 describes a method for increasing the thermal expandabilityof direct-injection foams by impregnating a prefoamed sheet with aninorganic gas, such as carbon dioxide, nitrogen, air or anotherpneumatogen, before heating to achieve the subsequent expansion, givingpolymer foam moldings of increased expandability.

WO-A 90/03998 discloses weathering-resistant polymeric moldingcompositions prepared by adding a polyalkylene glycol having a molecularweight of from 200 to 2000 to a) a matrix comprising a copolymer ofstyrene, acrylonitrile and from 0 to 50% by weight of methylmethacrylate, N-phenylmaleimide, N-C₁₋₄ -alkylmaleimide, maleicanhydride or a mixture thereof, and b) an impact modifying amount of agrafted-on EPDM rubber, a grafted-on alkyl acrylate rubber or a mixturethereof, in an amount sufficient to plasticize the polymeric molding andto reduce the discoloration of the polymer under weathering conditions.

None of these documents describes expandable styrene polymers having ahigh retention capacity for carbon dioxide.

It is an object of the present invention to provide an expandablestyrene polymer which contains carbon dioxide as blowing agent and hasan improved retention capacity for carbon dioxide. A further object ofthe present invention was to provide an expandable styrene polymer whichcan be impregnated with carbon dioxide more easily, i.e. in particularmore quickly.

We have found that, surprisingly, these objects are achieved by styrenepolymers which contain absorbers for carbon dioxide, also referred tobelow as carbon dioxide absorbers, mixed or copolymerized with theabove.

The present invention accordingly provides an expandable styrene polymerwhich contains a styrene polymer a), at least one carbon dioxideabsorber b), mixed or copolymerized with the above, in an amount of from0.01 to 50% by weight, based on the sum of a) and b), and a blowingagent or blowing agent mixture c) in an amount of from 0.5 to 12% byweight, based on the sum of a) and b), where the blowing agent orblowing agent mixture comprises from 10 to 100% by weight of carbondioxide (c₁) and from 0 to 90% by weight of at least one further blowingagent (c₂).

The present invention also provides a styrene polymer foam having adensity of from 0.005 to 0.2 g/cm² and containing at least one carbondioxide absorber b), mixed or copolymerized with the styrene polymer, inan amount of from 0.01 to 50% by weight.

The present invention also relates to the styrene polymer on which theexpandable styrene polymer according to the invention is based, whichcontains at least one carbon dioxide absorber b), mixed or copolymerizedwith the styrene polymer, in an amount of from 0.01 to 50% by weight,based on the sum of a) and b).

The present invention also provides an improved process for impregnatingstyrene polymers with carbon dioxide, in which a styrene polymer a)which contains at least one carbon dioxide absorber b), mixed orcopolymerized with the above, in an amount of from 0.01 to 50% byweight, based on the sum of a) and b), is impregnated with a blowingagent or blowing agent mixture c) which comprises from 10 to 100% byweight of carbon dioxide (c₁) and from 0 to 90% by weight of at leastone further blowing agent (c₂).

The carbon dioxide absorber b) is preferably employed in an amount offrom 0.02 to 30% by weight, particularly preferably from 1 to 20% byweight, based on the sum of a) and b).

As component a), the novel product contains polystyrene and/or a styrenecopolymer containing at least 50% by weight, preferably at least 80% byweight, of copolymerized styrene. Examples of suitable comonomers areα-methylstyrene, ring-halogenated styrenes, ring-alkylated styrenes,acrylonitrile, esters of acrylic or methacrylic acid with alcoholshaving 1 to 8 carbon atoms, N-vinylcarbazole, maleic acid and maleicanhydride. The styrene polymer advantageously contains a small amount ofa copolymerized branching agent, i.e. a compound containing more thanone, preferably 2, double bonds. The branching agent is generally usedin an amount of from 0.005 to 0.1% by weight, based on styrene.

It is also possible to use mixtures of different styrene polymers asdescribed, for example, in DE-A 3 901 329, 3 908 238, 3 936 596, 3 931862 and 3 916 602.

It is advantageous to use a styrene polymer having a viscosity (0.5%strength in toluene at 25° C.) of from 55 to 85 [ml/g], preferably from60 to 80 [ml/g], and a melt flow index MFI (190° C., 3.8 kp) of from 5to 30 [g/10 min], preferably from 7.5 to 20 [g/10 min]. The greatlyreduced melt flow index compared with conventional styrene polymershaving the same viscosity is characteristic of branched styrene polymerscontaining from 0.2 to 2.5, preferably from 0.2 to 1.0, branching pointsper 1000 monomer units (cf. DE-A 4 038 043).

The mean molecular weight of the styrene polymer is generally from150,000 to 400,000, preferably from 180,000 to 270,000. The proportionhaving a molecular weight of less than 200,000 is generally from 30 to80% by weight, preferably from 40 to 70% by weight. The component of thestyrene polymer which has a molecular weight of less than 200,000advantageously contains from 0.25 to 3, preferably from 0.5 to 2.5,branches per 1000 monomer units.

The expandable styrene polymers according to the invention are generallyin the form of small, discrete particles, for example beads ormicrogranules.

The novel styrene polymers a) and expandable styrene polymers contain,as the essential constituents, from 0.1 to 50% by weight, based on thesum of a) and b), of at least one carbon dioxide absorber b).

The carbon dioxide absorber according to the invention binds the carbondioxide physically and/or chemically.

Carbon dioxide absorbers which are suitable according to the inventioninclude ethylene carbonate, propylene carbonate, butoxydiethylene glycolacetate (CH₃ COO(CH₂ CH₂ O)₂ C₄ H₉), methoxydiethylene glycol acetate,methyl dimethoxyacetate and methyl and ethyl cyanoacetate.

Other suitable carbon dioxide absorbers are ketones, in particular4-methyl-2-pentanone, 2,6-dimethyl-4-heptanone,4-methyl-3-penten-2-one,2,4-pentanedione, acetoxyacetone,2,5-hexanedione, 4-hydroxy-4-methyl-2-pentanone and2-methoxy-2-methyl-4-pentanone.

Further carbon dioxide absorbers which are suitable according to theinvention are the compounds of the formula I ##STR1## in which,independently of one another, R¹ to R⁶ are identical to or differentfrom one another and are hydrogen or C₁ -C₁₀ -alkyl, and n is from 1 to1500.

Examples of these are polyethylene glycol, polyethylene glycol dimethylether, poly-1,2-propylene glycol and ethylene glycol monomethyl ether.Further examples are methyl isopropyl ethers of polyethylene glycolscontaining 2 to 8 --(CH₂ --CH₂ --O) units (for example Sepasolv®MPE fromBASF Aktiengesellschaft).

Due to the low water solubility, preference is given to compounds of theformula I in which at least one of the radicals R¹ and R² is C₁ -C₁₀-alkyl.

Polyethylene glycol dimethyl ether is particularly suitable. Thepolyethylene glycol dimethyl ethers employed according to the inventionhave a molecular weight of from 134 to 50,000, preferably from 500 to5000, and are commercially available.

Also suitable are poly-1,3-propylene glycol and the derivatives thereof,in particular the dialkyl ethers.

Polyethylenimines are likewise suitable according to the invention ascarbon dioxide absorbers.

A further group of carbon dioxide absorbers which are suitable accordingto the invention comprises amines. Tertiary amines are preferred, andhydroxyl-containing tertiary amines are very particularly preferred.Examples of these are dimethylethanolamine andN,N'-bis(2-hydroxyethyl)octadecylamine.

The carbon dioxide absorbers can be used alone or in mixtures. Of thegroups of carbon dioxide absorbers described above, preference is givenaccording to the invention to the compounds of the formula I.

The carbon dioxide absorbers can be introduced into the styrene polymerby mixing in the melt in an extruder with subsequent granulation or bymixing with the polymerization medium. However, the carbon dioxideabsorbers can also be incorporated into the polymer structure during thepolymerization in the form of appropriately substituted styrene monomersor comonomers.

The expandable styrene polymers contain a blowing agent or blowing agentmixture c) in an amount, based on the sum of a) and b), of from 0.5 to12% by weight, preferably from 1 to 10% by weight, particularlypreferably from 1.5 to 8% by weight. The blowing agent or blowing agentmixture c) comprises from 10 to 100% by weight of carbon dioxide c₁) andfrom 0 to 90% by weight of at least one further blowing agent c₂).

The expandable styrene polymers may contain, as the further blowingagent c₂), at least one C₃ - to C₈ -hydrocarbon, such as propane,butane, isobutane, n-pentane, i-pentane, neopentane, hexane, heptane oroctane. Preference is given to a commercially available pentane mixtureor n-octane. Particular preference is given to n-octane.

The effect of the invention is particularly pronounced if the blowingagent used is exclusively carbon dioxide.

Depending on the preparation of the styrene polymer containing thecarbon dioxide absorber, the impregnation with the blowing agent orblowing agent mixture c) is carried out during or after thepolymerization.

The styrene polymer particles may also contain other additives whichprovide the expandable products with certain properties, for exampleflameproofing agents based on organic bromine or chlorine compounds,such as trisdibromopropyl phosphate, hexabromocyclododecane andchlorinated paraffin, and synergists for flameproofing agents, such asdicumyl and highly reactive organic peroxides; furthermore antistatics,stabilizers, dyes, lubricants, fillers and substances which have anantiadhesive action during prefoaming, such as zinc stearate,melamine-formaldehyde condensates or silicic acid, and agents forshortening the demolding time during final foaming, for example glycerolesters or hydroxycarboxylic acid esters. Depending on the intendedaction, the additives may be distributed homogeneously in the particlesor may be in the form of a surface coating.

Other suitable additives are poly(2,6-dimethyl)-1,4-phenylene ether andpoly-1,4-phenylene sulfide (cf. DE-A-3 904 370 and DE-A-3 924 868).These additives increase the heat distortion resistance of the foam.Other suitable additives are styrene-soluble elastomers (cf. DE-A-3 915602), which increase the elasticity of the foam.

Other suitable additives are finely divided organic polymers having ahigh water-absorption capacity (cf. DE-A-4 014 261). These additivesimprove the flow properties.

In a preferred process, the expandable styrene polymer according to theinvention is prepared by polymerizing styrene, in the presence orabsence of comonomers, in aqueous suspension, it being possible to addthe above-described carbon dioxide absorber, blowing agent and anyadditives before, during or after the polymerization. Depending on thewater solubility of the carbon dioxide absorber, it may be advantageousto delay incorporation of the carbon dioxide absorber into the styrenepolymer a) until the latter has been melted in the extruder. In thelatter case, the impregnation with carbon dioxide is carried out in theextruder or by post-impregnation of the granules (for example by directgassing or in aqueous suspension).

It is usual to use from 0.005 to 1% by weight, preferably from 0.01 to0.75% by weight, in particular from 0.02 to 0.5% by weight, of aregulator having a chain-transfer constant K of from 0.1 to 50,preferably from 1 to 30, as described in EP-B-106 129 and DE-A-3 921148. Examples of suitable regulators are thiols, such as n-dodecylmercaptan (K=19), tert.-dodecyl mercaptan (K=3), n-butyl mercaptan(K=22) and tert.-butyl mercaptan (K=3.6), furthermore pentaphenylethane(K=2.0) and dimeric α-methylstyrene (K=0.5).

Suitable branching agents, which are generally used in amounts of from0.001 to 0.1% by weight, preferably from 0.005 to 0.05% by weight, aremonomers containing more than one, preferably two, polymerizable doublebonds, such as butadiene, isoprene, vinylcyclohexene, vinyl acrylate,divinylbenzene, glycol dimethacrylate, butanediol diacrylate, butanedioldimethacrylate and hexanediol diacrylate. The regulator and branchingagent are either introduced before commencement of the polymerization orare not added to the batch until during the polymerization, for exampleat a conversion of from 20 to 80%. In an advantageous procedure, theregulator is introduced before the polymerization and the branchingagent is added during the polymerization.

Simultaneous use of a regulator and branching agent gives a branchedstyrene polymer which has a conventional molecular weight and isdistinguished by a particularly high expansion capacity.

The content of free, un-copolymerized styrene in the expandable styrenepolymer should generally be low and is usually less than 0.2% by weight,preferably less than 0.1% by weight, in particular less than 0.08% byweight. In order to achieve this aim, it is expedient, if a mercaptan isused as the regulator, to delay addition thereof until during thepolymerization, at a conversion of from 20 to 90%.

The blowing agent-containing styrene polymer particles according to theinvention are preferably in bead form and generally have a diameter offrom 0.2 to 4 mm. They can be prefoamed by conventional methods, forexample using steam, to give foam particles having a diameter of from0.5 to 2 cm and a density of from 0.004 to 0.1 g/cm³.

The prefoamed particles can then be foamed to completion by conventionalmethods to give foam moldings having a density of from 0.005 to 0.2g/cm³.

EXAMPLES 1 to 10 and COMPARATIVE EXAMPLES 1 to 6

Carbon dioxide absorbers were used in Examples 1 to 10, while carbondioxide absorbers were not used in Comparative Examples 1 to 5. Thecarbon dioxide absorbers employed were dimethylethanolamine,polyethylene glycol dimethyl ether (mean molecular weight: 500) andN,N'-bis(2-hydroxyethyl)octadecylamine. In the Examples, parts andpercentages are by weight.

In Examples 1 to 5, the carbon dioxide absorber and in some casesn-octane were introduced before commencement of the polymerization. InExamples 6 to 10, the carbon dioxide absorber was admixed with thestyrene polymer, in some cases containing n-octane, in the melt in theextruder.

Preparation of the styrene polymers containing carbon dioxide absorber

A mixture of 150 parts of water, 0.1 part of sodium pyrophosphate, 100parts of styrene, 0.45 part of benzoyl peroxide and 0.15 part oftert.-butyl perbenzoate was heated to 90° C. with stirring in apressure-tight stirred reactor. In Examples 1 to 5, the amountsindicated in Table 1 of carbon dioxide absorber and n-octane wereadditionally introduced. In Example 8 and Comparative Example 4,n-octane in the amount indicated in Table 2 was additionally introduced.

After 2 hours at 90° C., 4 parts of a 10% strength aqueous solution ofpolyvinylpyrrolidone were added. The mixture was then stirred for afurther 2 hours at 90° C., subsequently for 2 hours at 100° C. andfinally for 2 hours at 120° C.

The bead polymer obtained, having a mean particle diameter of 1.0 mm,was isolated, washed and dried.

The bead polymers of Examples 6 to 10 and Comparative Examples 3 to 6(viscosity 75 ml/g), were plasticated in a heated twin-screw extruder(type ZSK 120) and extruded at a material temperature of 130° C. througha breaker plate (hole diameter 5 mm) into a water bath at 20° C. After aresidence time of 15 seconds, the polymer extrudate was fed to agranulator via deflection rolls. Examples 6 to 10 were carried out withaddition of 5% by weight, based on the sum of styrene polymer a) andcarbon dioxide absorber b), of a carbon dioxide absorber. Impregnationwith carbon dioxide

In each case, 200 g of the dried bead polymers of Examples 1 to 5 andComparative Examples 1 and 2 and the granules of Examples 6 to 9 andComparative Examples 3 to 5 were impregnated for 12 hours at 25° C. anda carbon dioxide pressure of 20 bar in a pressure-tight autoclave.Example 10 and Comparative Example 6 differed in that the impregnationwas carried out for 4 hours and 6 hours respectively.

Expandability

The expandability was determined using a metal-framed screen (mesh widthfrom 0.1 to 0.2 mm) measuring 1000×800×250 mm which was installed in asealed metal housing with steam inlet and outlet.

The steam at 105° C. was passed into the prefoaming apparatus from thebottom, passed through the mesh holding the products to be tested andescaped again through the steam outlet. Before the tests were commenced,the apparatus was first preheated for about 5 minutes. 100 g of theexpandable bead polymers having a particle diameter of from 0.5 to 1.5mm were subsequently distributed uniformly on the mesh, the apparatuswas closed and the steam valve was opened. After 6 minutes, the steamvalve was closed again and the metal housing was opened. After interimstorage for 12 hours, the prefoamed beads were again foamed for 6minutes in the prefoaming apparatus. The bulk density of the prefoamedmaterial was subsequently measured. The results (bulk densities) areshown in Tables 1 and 2.

The bulk density of the prefoamed, expandable styrene polymer of Example9 according to the invention and of Comparative Example 5 was measuredimmediately and after 1, 2, 3, 4 and 5 hours. The results are shown inTable 3.

It was found that the styrene polymer of Example 9, which containscarbon dioxide absorber, is impregnated significantly more easily (i.e.more rapidly) with carbon dioxide and releases the carbon dioxide itcontains significantly more slowly than does the styrene polymer ofComparative Example 5, which contains no carbon dioxide absorber.

In Example 10 according to the invention (5% by weight of polyethyleneglycol dimethyl ether having a mean molecular weight of 500), a bulkdensity of 60 g/l was achieved after an impregnation time of 4 hours. InComparative Example 6 (no carbon dioxide absorber), by contrast, a bulkdensity of only 294 g/l was achieved even after an impregnation time of6 hours.

                                      TABLE 1                                     __________________________________________________________________________                   Amount added                                                                  during the   Impregnation with CO.sub.2                                                                  Bulk                                Example                                                                            Carbon dioxide                                                                          polymerization                                                                        n-octane                                                                           CO.sub.2 pressure                                                                    Temperature                                                                          density                             No.  absorber  [parts] [%]  [bar]  [°C.]                                                                         [g/l]                               __________________________________________________________________________    1    Dimethylethanol-                                                                        5       --   35     25     39                                       amine                                                                    2    Dimethylethanol-                                                                        5       1    20     25     24                                       amine                                                                    3    N,N'-bis(2-hydroxy-                                                                     5       --   10      0     42                                       ethyl)octadecyl-                                                              amine                                                                    4    Polyethylene                                                                            5       --   10     25     86                                       glycol dimethyl                                                               ether                                                                    5    Polyethylene                                                                            5       1    10     25     36                                       glycol dimethyl                                                               ether                                                                    1     --       --      --   10     25     127                                 (comp.)                                                                       2     --       --      --   20     25     101                                 (comp.)                                                                       __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                       Amount added                                               Example            in the extruder                                                                       n-octane                                                                           Bulk density                                  No.  Carbon dioxide absorber                                                                     [parts] [%]  [g/l]                                         __________________________________________________________________________    6    N,N'-bis(2-hydroxyethyl)-                                                                   5       --   73                                                 octadecylamine                                                           7    Polyethylene glycol dimethyl                                                                5       --   25                                                 ether                                                                    8    Polyethylene glycol dimethyl                                                                5       2    49                                                 ether                                                                    3     --           --      --   223                                           (comp.)                                                                       4     --           --      2    111                                           (comp.)                                                                       __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                        Time after  Bulk density                                                      impregnation                                                                              [g/l]                                                             [hours]     Example No. 9                                                                             Example No. 5 (Comp.)                                 ______________________________________                                        0            60         171                                                   1            85         224                                                   2           123         342                                                   3           160         406                                                   4           207         456                                                   5           240         495                                                   ______________________________________                                    

We claim:
 1. An expandable styrene polymer containinga) a styrenepolymer, b) at least one carbon dioxide absorber, wherein the carbondioxide absorber is a compound of the formula I ##STR2## in which R' toR⁶ are identical to or different from one another and are hydrogen or C₁-C₁₀ -alkyl, and n is from 1 to 1500 or an amine, mixed or copolymerizedwith the above, in an amount of from 0.01 to 50% by weight, based on thesum of a) and b), and c) a blowing agent or blowing agent mixture in anamount of from 0.5 to 12% by weight,where the blowing agent or blowingagent mixture comprises c₁) from 10 to 100% by weight of carbon dioxideand c₂) from 0 to 90% by weight of at least one further blowing agent.2. An expandable styrene polymer as claimed in claim 1, wherein at leastone of the radicals R¹ and R² in the compound of the formula I is C₁-C₁₀ -alkyl.
 3. An expandable styrene polymer as claimed in claim 2,wherein the carbon dioxide absorber is a polyalkylene glycol dimethylether having a molecular weight of from 134 to 50,000.